When a sheet-shaped work is bent by installing a die having a V-shaped bending groove and a punch having a tip end formed into a shape corresponding to the bending groove to a bending machine such as a press brake, there are processing methods including air bending (free bending), bottoming (pressure bending), coining (high pressure bending), and the like.
The air bending is the processing method configured to bend a sheet-shaped work into a V shape by pressuring and bending the work with the tip end portion of the punch, the work being supported at two points respectively of two shoulders on the V-shaped bending groove in the die. This air bending can bend the work at various desired angles by using combinations of the punch and the die, but has a problem of a large spring-back amount.
In bending a work by the bottoming, a work is sandwiched and pressed between the V-shaped bending groove in the die and the punch. However, when a work is bent at, for example, 90°, this method is configured to use the die having an angle of the V groove, for example, equal to 88° or 89° so as to perform bending to form 90° as the work shows spring-back. In this case, although the spring-back amount is smaller than the case of the air bending, the spring-back amount is not always stabilized constantly. Accordingly, there is a problem for bending a work at higher accuracy.
In bending a work by the coining, a high pressure is applied to the work, the pressure being about 5 to 8 times greater than the case of the air bending so as to eliminate the spring back of the work, and thereby to transfer the shapes of the punch and the die to the work. As a result, a frame of the bending machine is apt to cause large strain, and accordingly there is a problem that it is necessary to increase the frame rigidity.
Here, a precedent example concerning the bending machine to perform bending by air bending and coining is disclosed in JP-A 2001-1049 (Patent Document 1). Meanwhile, precedent examples concerning the air bending are disclosed in JP-A 8-155553 (Patent Document 2) and JP-A 7-39939 (Patent Document 3).
According to the invention disclosed in Patent Document 1, when a work is subjected to coining by use of a bending machine such as a press brake, a ram is descended until the value of a deviation counter exceeds a threshold for coining, which is stored in an NC device in advance, and the descending action of the ram is terminated when the value exceeds the threshold by considering a position of the ram at this time as a bottom dead center of the coining ram. Accordingly, there may be a case where the coining is terminated at a degree of a bending work corresponding to the bottoming, and there is a problem that the coining may be terminated without providing the work with a pressure sufficient to transfer the shapes of the die and the punch to the work.
The invention according to Patent Document 2 is premised on bending a work by the air bending or the bottoming, and does not have an assumption on bending by the coining configured to apply a high pressure to the work, the pressure being 5 to 8 times greater than that of the air bending.
Specifically, suppose the concept of bending a work by controlling an engagement positional relation between the punch and the die as similar to the air bending or the bottoming is applied to the coining. This leads to a problem such as a shortage of pressure application or breakage of the die due to an excessive pressure application along the variation in thicknesses of the works attributable to, for example, manufacturing errors. This is because the coining is configured to apply a high pressure to a work in order to transfer the shapes of the punch and the die to the work.
The invention according to Patent Document 3 is aimed at improving accuracy of the bending angles of multiple points of a work by measuring angles at two points on left and right sides of the work while the work is detached from a die after a first session of bending is completed, and by calculating stroke correction amounts for left and right drive shafts based on differences from respective target angles when the measured angels do not match the target angles, and then by performing correction. However, the invention according to Patent Document 3 relates to the air bending, and is configured to perform positional control of a ram. Accordingly, it is difficult to apply this technique directly to the bottoming or the coining.
Incidentally, the air bending is also referred to as three-point bending because it is possible to change a bending angle for a work in terms of a positional relation among three points of two shoulders of the V-shaped bending groove in the die and the tip end of the punch.
This bending method has a problem of a large spring-back amount of the work. The bottoming is configured to sandwich the work between inclined planes of the V groove in the die and inclined planes of the punch. Although the spring-back amount is reduced, there is a problem with controlling the bending angle for the work more accurately. The coining is configured to apply a higher pressure (5 to 8 times greater than the case of the air bending) to the work after sandwiching the work between the inclined planes of the V groove in the die and the inclined planes of the punch, and is thus possible to process the angle for the work accurately, but has a problem that the rigidity of the frame of the bending machine (the press brake) must be increased.
Particularly, in the invention according to Patent Document 1, when a sheet-shaped work is bent into the V shape by use of the die having the V-shaped bending groove and the punch, the air bending and the coining are performed by using the same die. Accordingly, at the time of coining, the work is strongly high-pressured (pressurized) with all the surfaces of the inclined planes of the bending groove in the die and the inclined planes on the tip end side of the punch. Thus, the pressure at the time of the coining becomes extremely high, the pressure for transferring, to the work, the inclined angle of the inclined planes of the bending groove in the die and the inclined angle of the inclined planes on two sides on the tip end of the punch. For this reason, it is necessary to increase the frame rigidity.
In the meantime, a configuration of a die which appears to be similar at the glance to the die according to the present invention is disclosed in JP-A 9-295052 (Patent Document 4). The invention described in this Patent Document 4 is based on aspects that the rate of progression of initial wear is not stabilized quickly if a curvature radius of a shoulder of a die is as small as about 0.8 mm, and that the wear at the shoulder of the die is stabilized when works are bent 2000 times or more, for example, and is configured to form a curved surface at the shoulder of the die along a curved line equivalent to the shoulder after bending 2000 times. The curvature radius at that point becomes equal to 0.99 mm, which is quite small.
In other words, the invention according to the aforementioned publication of patent application does not intend to form the portion of a V-shaped bending groove in the die above the approximately intermediate depth position into a convex curved surface having a large radius of curvature, for example, and does not relate to the present invention at all.
Moreover, in the bottoming, the spring-back still exists though the spring-back amount is smaller than that of the air bending. Accordingly, when a target bending angle is set, for example, equal to 90°, the inclined angle of the V-shaped bending groove in the die is set to a slightly smaller angle than 90° (such as 88°) in expectation of the spring-back amount. On the other hand, the coining is the process to transfer, to a work, the inclined angles of the bending groove (the V groove) in the die and of the punch, and is configured to apply a high pressure to the work. For this reason, in an attempt to bend a work accurately at an angle of, for example, 90° by the coining, the use of the die used for the air bending or the bottoming may result in bending at an angle equal to the angle that is preset in expectation of the spring-back amount. Accordingly, it is not possible to achieve bending at an angle targeted originally.
Therefore, in order to bend a work by the coining, it is necessary to form the angle of the V groove in the die and tip end angle of the punch to the target angles (such as 90°) in advance. Since the coining is configured to transfer, to the work, the angle of the V groove in the die and the tip end angles of the punch, it is possible to bend the work accurately, and thus the coining is desirable. However, as mentioned previously, this is configured to apply the high pressure (5 to 8 times greater than the case of the air bending), and there is a demand for achieving the coining with less pressure.
The present invention has been made to solve the foregoing problems, and a first object of the present invention is to provide a bending method, a die and a bending machine used for the bending method, which are capable of applying a pressure to a work without excess or deficiency, and accurately performing a bending process to transfer shapes of a punch and a die to the work.
A second object of the present invention is to provide a bending method, a die and a bending machine used for the bending method, which are capable of reducing a pressure for bending by applying a pressure to a work locally.
A third object of the preset invention is to provide a bending method, a die and a bending machine used for the bending method, which are capable of suppressing a pressure to a small amount by reducing a bending-back amount of a work.